A standard drill chuck has a chuck body comprised of one part formed with a frustoconical surface centered on the chuck axis and another part axially displaceable relative thereto and forming an abutment surface generally perpendicular to the chuck axis. Three angularly equispaced jaws are axially braced between the centering and abutment surfaces and are urged radially apart by springs. The two parts are threaded together so the surfaces can be axially relatively displaced, thereby radially displacing the jaws. To hold a tool, the parts are relatively rotated to separate the surfaces and spread the jaws, the shank of the tool is placed between the spread jaws, and the parts are oppositely relatively rotated to clamp them on the shank of the tool.
When such an arrangement is used for drilling, the chuck body is threaded onto the rotary spindle of the drill transmission so that the drill bit held between the jaws is centered on and rotated about the chuck axis. When, however, the chuck is used to hold a tap for threading predrilled holes, a cardan or universal joint permitting limited displacement of the drill axis relative to the spindle axis is provided in a two-part chuck body so that the tap held in the chuck can center itself in the predrilled hole. In, for instance, a production setup with a drill-press arrangement for tapping predrilled holes such an arrangement makes it irrelevant if the spindle axis is slightly out of line with the hole axis, whereas without the cardan joint the hole would be tapped offcenter.
It is therefore necessary to dispose of two different chuck assemblies to use the same piece of equipment both for tapping and for drilling. The limited swiveling of the tapping chuck makes it unusable for accurately drilling holes, while the inability to swivel makes the drilling chuck unsuitable for tapping predrilled holes.